The removal of noxious gases and toxic emissions from combustion systems particularly municipal solid waste incinerators and refuse to energy plants has received much attention during the past decade as public concern over the environmental impact as such emissions has increased. Dioxins and their role in polluting the environment have received particular attention as they have been recognized as causing serious health hazards. It has long been recognized that the removal of these noxious gases is desirable however no efficient and effective method or apparatus has been accepted in the industry as the solution to the mounting problems caused by these noxious emissions.
Noxious gas removal systems are of various types and many are available today. The most commonly used systems are wet or dry scrubbers which remove acid gases such as HCl and SO.sub.2 from the flue gas as it exits the boiler or back end of the system. However, these systems are capital intensive to install, costly to operate and maintain, require significant energy during the reaction or removal and require excessive quantities of any sorbent that would be used.
In an attempt to avoid the requirement for wet or dry scrubbers, several systems have been developed which inject the sorbent directly into the combustion chamber either with the fuel or injected separately. Each of these systems has demonstrated varying degrees of success but no direct injection system has been able to achieve 90 percent HCl and 70 percent SO.sub.2 removal as required in states such as New Jersey, California and Connecticut.
The prior art is replete with various processes for the treatment of flue gases and includes for instance U.S. Pat. No. 4,245,573 to Dixit et al. directed to the use of an additive such as MgO.Mg silicate injected into flue gas stream.
U.S. Pat. No. 4,185,080 issued to Rechmeier discloses the use of limestone or dolomite being added to cold dust furnace or blasted into the furnace together with fuel oil.
In U.S. Pat. No. 4,253,408, issued to Kramer, additive materials that include calcium oxide are combined with the sewage to prevent corrosion in the incinerator.
In the Hughes U.S. Pat. No. 4,159,683, a basic additive, sodium bentonite, is added to the furnace directly to reduce soot and slag but the high temperatures to which the additive is subjected in the combustion chamber would be too high for the desired effective reaction to take place.
Calcium oxide is suggested in the Roma Pat. No. 2,800,172 to be added to the combustion air to limit slag formation. Again the temperature would be too high to attain the effective reaction.
An interesting method of removal of SO.sub.2 is disclosed in the publication Chemical Engineering, Feb. 20, 1984, pp. 30-33, 35. This method proposes the use of limestone, dolomite or lime to achieve its hoped for goal. The thrust of the article appears to propose the development of a technique to control furnace temperatures so that the limestone does not experience "dead burn".
The principal reasons why none of the prior art methods found favor were numerous but they did include (a) the thermodynamic equilibrium did not favor the desired reactions that were to take place, (b) the rates of reaction were much too slow to permit sufficient amounts to react, (c) the temperature of the injection of whatever sorbent was used for either too high or too low to achieve the reaction sought, (d) the sorbents at times became incapacitated due to dead burn and/or were inherently unable to sorb the pollutants due to its lack of porosity, and (e) the reactants/products expansion coefficient.
It is believed that to date there are no proven techniques or prior art processes which prevent the formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in solid waste incinerators or resource recovery systems.
It is currently believed that the primary source of the PCDD and PCDF found in the flue gas of incinerated waste is caused by chloride donations by highly reactive chemicals such as HCl and Cl.sub.2 to certain chlorinated organic chemical precursors such as the man-made chlorinated phenols and polychlorinated biphenyls (PCB) and non-chlorinated organic chemical precursors such as lignin as the flue gas cools after combustion.
It has been thought in the prior art that if the PCDD and PCDF precursor chemicals can be completely oxidized in the combustion process PCDD and PCDF will not form. However, there are no existing municipal solid waste incinerators or resource recovery facilities at which this theory has been demonstrated.
It now has been found that it is possible to prevent dioxins such as PCDD and furans such as PCDF from forming by removing potential chloride donors such as HCl and Cl.sub.2 from the combustion gases. Provided that the temperatures of the gases in the combustion chamber are controlled within specified limits, it has been found that highly reactive carbonated sodium, a term generic to sodium bicarbonate and sodium carbonate, can be utilized to react with the potential chlorine donors to form stable salts before these chloride donors can react with the PCDD and PCDF precursor chemicals to form dioxins and furans.